The echinocandins are novel anti-fungal medicaments, which belong to 6-member ring compounds containing acetyl. Echinocandins are medicaments for inhibiting glucan synthetase and exert germicidal effect by non-competitively inhibiting the synthesis of -β(1,3)-D-glucan in cell-wall of fungi. Glucan is a polysaccharide in cell-wall of fungi, which is the important component in cell-wall and can maintain the integrity of cell-wall and the stability of osmotic pressure.
Caspofungin, the structure of which is shown in formula I, is the first anti-fungal medicament in echinocandins.

Caspofungin possesses broad-spectrum antifungal activities, demonstrating excellent antifungal activities toward fungi of Candida albicans, non-Candida albicans and aspergillus, and in vitro antifungal activities toward fungi, such as Candida and aspergillus etc., resistant to fluconazole, amphotericin B or fluorocytosine. Caspofungin does not possess cross-resistance with azoles or polyenes, no natural drug resistance exists in the isolates of Candida, and caspofungin applies to invasive aspergillosis to which other treatment is inefficient or intolerable.
Caspofungin preparation was firstly developed by Merck Sharp & Dohme (USA), marketed with the tradename “Cancidas”, and in the preparation, caspofungin is administered in the form of caspofungin diacetate. Caspofungin has low stability, and is prone to degradation, thereby producing many degradation impurities. The degradation impurity at RRT (relative retention-time) 1.35 is a major impurity. And for the medication safety, the content of the impurity in “Cancidas” should be not more than 0.2% according to the quality standard of Cancidas stipulated by Merck Sharp & Dohme.
In U.S. Pat. No. 5,952,300 and U.S. Pat. No. 6,136,783, caspofungin pharmaceutical compositions comprising acetate buffer system and excipient and the indications thereof have been disclosed. In U.S. Pat. No. 5,952,300 and U.S. Pat. No. 6,136,783, it is disclosed that caspofungin pharmaceutical composition is stable due to the use of acetate buffer instead of tartrate buffer. However, the stability and degradation impurities for the composition have not been characterized or defined in detail. In US2010137197, another caspofungin pharmaceutical composition with better stability has been disclosed. The composition comprises one or more of non-reducing sugar with glass transition temperature over 90° C. and acetate buffer system at pH 5-7, therefore, the composition is better than the composition disclosed in U.S. Pat. No. 5,952,300 or U.S. Pat. No. 6,136,783 in stability. However, in US2010137197, the degradation impurity at RRT (relative retention-time) 1.35 from caspofungin (impurity RRT 1.35) has not been characterized or defined. According to the PDA (absorption spectrum) pattern, the degradation impurity RRT 1.35 has characteristic absorption peaks as shown in FIG. 4. Such peaks are similar to the characteristic absorption peaks of the compound of formula I as shown in FIG. 5.
In US20090324635, caspofungin free of impurity A (formula II), and preparation method thereof have been disclosed. In US20090291996, caspofungin free of impurity C0 (formula III), preparation method and the pharmaceutical composition thereof have been disclosed. The degradation impurity of formula II and impurity RRT 1.35 have not been characterized or analysed in both of the documents whose contents relate to raw medicine. In US20090170753, another stable pharmaceutical composition comprising caspofungin has been disclosed, which comprises additional pH adjusting agent for caspofungin salt, the amount of which is less than 0.3 mole equivalent, and pharmaceutically acceptable amount of excipients for efficiently forming lyophilized cake. It is believed that the composition has better stability due to the minor amount of additional acetate pH adjusting agent. In US20090170753, the degradation impurity CAF-42 was determined, while impurity RRT 1.35 has not been characterized or defined. In CN102166186A, another caspofungin injectable composition, which is more stable, has been disclosed. The composition has better stability due to the existence of sorbitol or the mixture comprising sorbitol and other excipients. However, the inventors have tested the composition, and found that the stability of that composition is not as good as it was alleged and much lower than that of the formulation disclosed herein.

Impurities in an active pharmaceutical ingredient, such as caspofungin, are not desired, which may even do harm to the patient. However, it is impossible to remove all of the impurities. Therefore, it is important for a developer in pharmaceutical formulation field to reduce the content of impurities.
However, all of the pharmaceutical compositions known in the prior art are not desirable formulations, in which impurity RRT 1.35 is not strictly controlled. Therefore, it is necessary to develop a novel pharmaceutical composition and the preparation process thereof for reducing the HPLC content of impurity RRT 1.35 in caspofungin, improving the safety and stability thereof, and extending the shelf-life of medicament.
Upon lots of experiments, great advancements have been obtained in the stability of caspofungin pharmaceutical composition by the inventors.
In the present invention, a lyophilized pharmaceutical composition and the preparation method thereof have been provided. Said composition is of low-impurity, safe, stable, and reproducible, and can be directly used for treating/preventing fungal infections.